Shock is a state of global tissue hypoxia. Pulse oximetry is widely used in anesthesia and critical care medicine to provide noninvasive information about arterial oxygen saturation (SaO2). The oximeter uses either the transmission or reflectance of near-infrared light to monitor the changes in the hemoglobin spectrum with oxygenation and deoxygenation. Oximetry probes are usually applied to a patient's fingers or ears for convenience, but this has a signal acquisition failure rate of 1.12% to 2.5% (Reich et al., Anesthesiology 84:859-864 (1996); Severinghaus et al., Anesthesiology 76:1018-1038 (1992); Moller et al., Anesthesiology 78:445-453 (1993)), and more accurate readings can be obtained from better-perfused superficial tissues, such as the cheek (O'Leary et al., Anesth. Analg. 75:495-498 (1992)), nasal septum (Ezri et al., J. Clin. Anesth. 3:447-450 (1991)), and tongue (Jobes et al., Anesth. Analg. 67:186-188 (1988)). More recently, oximeters have been placed into deep, vessel-rich areas, such as the esophagus (Vicenzi et al., Crit. Care Med. 28:2268-2270 (2000)), pharynx (Keller et al., Anesth. Analg. 90:440-444 (2000); Brimacombe et al., Can. J. Anaesth. 47:907-909 (2000)), and trachea (Brimacombe et al., Anesth Analg. 91:1003-1006 (2000)) where, because the heart lies in close proximity to the esophagus, esophageal oximetry readings seem to provide more accurate readings than surface oximetry, even in hypoperfusion states. Appropriately located and directed esophageal oximetry probe have been used to derive oximetry readings from specific ventricular locations within the heart (Margreiter et al., Anesth. Analg. 94:794-798 (2002)). The resulting transesophageal echocardiography (TEE) permits the measurement of cardiac performance during surgery, but it involves passing an endoscope into the esophagus of the patient and the use of ultrasound imaging technology (Bryan et al., Ann. Thorac. Surg. 59:773-779 (1995)).
The mixed venous oxygen saturation (SVO2) of blood in the pulmonary artery has emerged as an important monitor of the balance between oxygen delivery and oxygen demand. In a prospective randomized study of critically ill patients, a direct comparison of a resuscitation strategy based on the normalization of cardiac index and one based on the normalization of mixed venous saturation were reported to have equivalent efficacy in predicting a clinical course of treatment (Gattinoni et al., N. Engl. J. Med. 333:1025 (1995)). Several studies have shown that mixed venous oxygen saturation has significant prognostic value in the perioperative period (e.g., Svedjeholm et al., Eur. J. of Cardio-Thor. Surg. 16:450 (1999)).
The determination of mixed venous saturation requires the measurement of the oxygen saturation of blood from the pulmonary artery. This can be accomplished using a fiber-optic oximeter coupled to a pulmonary artery catheter (Baele et al., Anesth. Analg. 61:513 (1982)). The oximeter uses the reflectance of near-infrared light to monitor the changes in the hemoglobin spectrum with oxygenation and deoxygenation. One commercially available fiber optic oximeter (Abbott Laboratories, Abbott Park, Ill.) measures the reflectance of three wavelengths of light (670, 700, 800 nm) and applies an empirically derived polynomial function to calculate saturation (Zilstra et al., In Visible and Near Infrared Absorption Spectra of Human and Animal Haemoglobin, Utrecht: VSP, pp. 262 (2000)).
Nevertheless, there are no techniques currently available for continuous noninvasive measurement of the oxygen saturation of blood flowing through the heart. Placement of a pulmonary artery catheter is an invasive procedure with many possible complications. These include bleeding, infection, pneumothorax, arrhythmia, and pulmonary artery rupture (see Table 1). Puri et al., (Crit. Care Med. 8: 495, (1980)) found at least a 10% incidence of complications with the placement of pulmonary artery catheters by critical care fellows.
TABLE 1Common ComplicationsIncidenceReferenceMinor transient arrhythmias4.7%-69%Sise et al., Crit. Care Med. 9:315 (1981);Transient VT or VF0.3%-63%Katz et al., JAMA 237:2832 (1977); Boyd etOverall risk of transient arrhythmia48%al., Chest 84:245 (1983).Right Bundle Branch Block0.1%-4.3%Fowler et al., Am. Heart J. 46:652 (1951);Shah et al., Anesthesiology 61:271 (1984);Morris et al., Arch. Intern Med. 147:2005(1987); Patil, Crit. Care Med. 18:122(1990); Sprung et al., Crit. Care Med. 17:1(1989)Complete heart block (in patients with<1%Shah et al., 1984; De Lima et al., J.pre-existing LBBB)Cardiothoracic Vasc. Anesth. 8:70 (1994)VT (with hypotension) associated with2%Baldwin et al., Heart Lung 29:155 (2000).removal of PACInfection at insertion site0%-22%Michel et al., JAMA 245:1032 (1981);Mermel et al., Am. J. Med. 91:197S (1991).Catheter related sepsisa0%-2%Shah et al., 1984; Patil, 19903Mural thrombusb28%-61%Patil, 1990; Sprung et al., 1989Pulmonary infarction0.1%-7%Boyd et al., 1983; Foote, et al., N. Engl. JMed. 290:927 (1974); Elliott et al., Chest76:647 (1979).Uncommon ComplicationsPulmonary artery false aneurysm0.06%-0.2%De Lima et al., 1994; Robin, Chest 92:727(1987).Pulmonary artery rupture<1%Shah et al., 1984; McDaniel et al., JThorac. Cardiovasc. Surg. 82:1 (1981)Pneumothorax<1%Sise et al., 1981; Shah et al., 1984Cardiac tamponade due to catheter<1%Sise et al., 1981; Shah et al., 1984, Elliott etinduced perforational., 1979; Greenall et al., BMJ 2:595 (1975)Catheter knotting<1%Schwartz et al., JAMA 237:113 (1987)Valvular damage<1%Boscoe et al., BMJ 283:346 (1981);O'Toole et al., NEJM 301:1167 (1979).aRisk increases significantly when catheter is in place >3-4 days. bSignificance unknown 
Due to the high potential for complication, the placement of pulmonary artery catheters is often delayed or foregone altogether. A recent prospective randomized trial studied the use of early goal-directed therapy in patients with septic shock begun in the emergency room, and found a decrease in hospital mortality from 46.5% to 30.5% (Rivers et al., N. Engl. J. Med. 345:1368 (2001)). However, in that study, they could not measure mixed venous saturation, as the insertion of pulmonary artery catheters was impractical. Instead the investigators used the poor surrogate of central venous saturation as a guide for resuscitation (Edwards et al., Crit Care Med. 26:1356 (1998)). Perhaps early intervention would have proven to be even more beneficial if mixed venous saturation could have quickly and easily been measured.